Traditionally, pharmaceuticals have primarily consisted of small molecules that are dispensed orally (as solid pills and liquids) or as injectables. Over the past three decades, formulations (i.e., compositions that control the route and/or rate of drug delivery and allow delivery of the therapeutic agent at the site where it is needed) have become increasingly common and complex. Nevertheless, many questions and challenges regarding the development of new treatments as well as the mechanisms with which to administer them remain to be addressed.
Although considerable research efforts in this area have led to significant advances, drug delivery methods/systems that have been developed over the years and are currently used, still exhibit specific problems that require improvement. For example, many drugs exhibit limited or otherwise reduced potencies and therapeutic effects because they are either generally subject to partial degradation before they reach a desired target in the body, or accumulate in tissues other than the target, or both.
One objective in the field of drug delivery systems, therefore, is to deliver medications intact to specifically targeted areas of the body through a system that can stabilize the drug and control the in vivo transfer of the therapeutic agent utilizing either physiological or chemical mechanisms, or both. Over the past decade, materials such as polymeric microspheres, polymer micelles, soluble polymers and hydrogel-type materials have been shown to be effective in enhancing drug stability in vitro and in vivo, release dynamics, targeting specificity, lowering systemic drug toxicity, and thus have shown great potential for use in biomedical applications, particularly as components of various formulations and drug delivery devices.
Therefore a need exists in the biomedical field for low-toxicity, biodegradable, biocompatible, hydrophilic polymer conjugates comprising pharmaceutically useful modifiers, which overcome or minimize the above-referenced problems. Such polymer conjugates would find use in several applications, including therapeutic and diagnostic pharmaceutical formulations, gene vectors, medical devices, implants, and other therapeutic, diagnostic and prophylatic agents.
The design and engineering of biomedical polymers (e.g., polymers for use under physiological conditions) are generally subject to specific and stringent requirements. In particular, such polymeric materials must be compatible with the biological milieu in which they will be used, which often means that they show certain characteristics of hydrophilicity. In several applications, they also have to demonstrate adequate biodegradability (i.e., they degrade to low molecular weight species. The polymer fragments are in turn metabolized in the body or excreted,).
Biodegradability is typically accomplished by synthesizing or using polymers that have hydrolytically unstable linkages in the backbone. The most common chemical backbone components with this characteristic are esters and amides. Most recently, novel polymers have been developed with anhydride, orthoester, polyacetal, polyketal and other biodegradable backbone components. Hydrolysis of the backbone structure is the prevailing mechanism for the degradation of such polymers. Other polymer types, such as polyethers, may degrade through intra- or extracellular oxidation. Biodegradable polymers can be either natural or synthetic. Synthetic polymers commonly used in medical applications and biomedical research include polyethyleneglycol (pharmacokinetics and immune response modifier), polyvinyl alcohol (drug carrier), and poly(hydroxypropylmethacrylamide) (drug carrier). In addition, natural polymers are also used in biomedical applications. For instance, dextran, hydroxyethylstarch, albumin, polyaminoacids and partially hydrolyzed proteins find use in applications ranging from plasma expanders, to radiopharmaceuticals to parenteral nutrition. In general, synthetic polymers may offer greater advantages than natural materials in that they can be tailored to give a wider range of properties and more predictable lot-to-lot uniformity than can materials from most natural sources. Methods of preparing various polymeric materials are well known in the art. In many biomedical applications, polymer molecules should be chemically associated with the drug substance, or other modifiers, or with each other (e.g., forming a gel). Several properties of the final product depend on the character of association, for example, drug release profile, immunotoxicity, immunogenicity and pharmacokinetics. Therefore a need exists for methods of polymer association with drug substances and other pharmaceutically useful modifiers that would be compatible with the biomedical use of the associates (conjugates, gels). Such methods should further allow, where necessary, drug release in under physiological conditions with an optimal rate and in a chemical form or forms optimally suited for the intended application.